1. Field of the Invention
This invention relates to a vibratory multi-wire type method for cutting a hard and brittle material with fine lines of wire applying slurry and to an apparatus for carrying out the method.
2. Description of the Prior Art
A method has been known for carrying out a cutting process, such as multi-line grooving, cutting off, etc., on a hard and brittle material such as a semi-conductor, a glass material and a ceramic material by means of fine wires. An apparatus employed in carrying out this method has been also known by the name of a multi-wire saw. The multi-wire saw is generally arranged to have many lines of a continuous fine wire stretched and wound or lapped many times around a plurality of multi-grooved blocks; to bring the lower horizontal parallel parts of the stretched lines of the wire into pressed contact with an object to be processed; to allow these fine wire lines to travel while applying an abrasive grain suspension to the contact parts between the wire and the object; and thus to carry out a cutting process such as multi-wire grooving, cutting off, etc. Since the multi-wire saw uses fine wire lines which permit having a smaller allowance for cutting, it gives an improved yield and excels beyond other mechanical cutting methods in that it causes the generation of an affected layer in a much lesser degree that in other methods. According to the arrangement of the multi-wire saw, tightly stretched fine wires are caused to travel while in a state of being pressed against the object to be processed while a suspension containing a granular abrasive is being applied to the part which is in pressed contact to cut the object by lapping between the wire and the object. This arrangement tends to cause a breaking of the fine wire. Wire breakage during a cutting process not only lowers the precision of the cutting work but also greatly lowers the productivity of the cutting work as it requires much time and labor for rearrangement of the wire lines. To solve this problem, a method has been proposed in which the wire lines are allowed to travel at a relatively low speed to prompt a granular abrasive to enter into a cutting part by imparting vibration either to the wires or to the object to be processed. However, at present, the details of the interrelation between the travelling speed of the wires and the amplitude and frequency of the vibration for the processing efficiency, wire breakage and the cutting precision still remain unknown. Therefore, the above-stated method of imparting vibration has not reached a practicable stage as yet, thus leaving some points in need of further improvement.
In an attempt to improve the productivity and cutting precision of the above-stated multi-wire saw and to prevent the wire breakage, the present applicant has conducted research and experiments in varied manners. The findings obtained through these experiments are as follows: The cutting work efficiency (cutting amount per unit time) generally increases with an increase in the force with which the fine wire lines are kept in contact with the object being processed. However, the probability of wire breakage increases when the pressing force is excessive. Therefore, to increase the machining work efficiency while preventing wire breakage, it is necessary to keep the wire tension at about 2 kg and to limit the above-stated pressing force to between 40 and 70 g per wire line in a case where, for example, commercially available wire measuring 0.2 mm in diameter is arranged to travel at about 80 to 120 m/min. In such a case, it has been found that the processing efficiency can be improved by imparting vibration in a specific direction to the wire stretched and wound many times around the multi-grooved blocks.
Further studies on the impartment of vibration has revealed that great improvement can be attained in the process efficiency when the wire and the cutting faces of the object arranged to partially or totally come into contact and move away from each other in a repetitive manner while an abrasive grain suspension (slurry) is being applied to the contacting faces of the wire and the object.
The five wire lines stretched and wound around the multi-grooved blocks must be kept travelling at a speed of about 80 to 120 m/min at their parts which are in pressed contact with the object. However, they must be moved away from the cutting parts, as a used wire portion, before they reach an allowable limit of wear set for an easily breakable wire. Therefore, in order to attain a desired machining efficiency without incurring wire breakage, the following technical requirement must be satisfied: For example, new wires must be gradually fed to the cutting parts at a rate of 0 to 4 m/min while moving the used wire portion away from the cutting parts. Meanwhile, with the wire lines kept at a predetermined value of tension, the wire lines must be allowed to travel at their cutting parts at a rate of 80 to 120 m/min.